It looks like you have them well sited in full sun. Do you have a sprinkler system that hits them with water?

My first instinct is that you are pruning them incorrectly. Ninebark should not be sheared regularly. Not only does shearing cause them to become thick and congested, you are also spreading the powdery mildew spores from leaf to leaf every time you shear them.

They need to thinned out to encourage airflow. I would recommend pruning soon, right after the shrub stops blooming. 

Here is a pruning guide from the University of Minnesota. It says to prune ninebark in late winter, but it doesn't necessarily matter. Since you are fighting a disease, I would say to do it soon. Make sure you do it on a dry day. Use rubbing alcohol on your tools when you switch between plants (so you don't spread another disease between the two shrubs).

Start by removing any diseased or dead branches, then begin to cut out branches at the base of the shrub. Start with some of the oldest branches first. They will be thicker and the bark might be more shaggy. Remove the oldest branches as close to the ground as you can go. 

Do not cut out more than one third of the branches in a year. "Renovating" these shrubs is a continual process. Plan on removing the oldest branches every year or two. These "thinning cuts" will help to open up the shrub and promote good air flow and new growth. 

During the rest of the season, be intentional about pruning for size. Don't take off more than six inches off the tips of the branches at a time and cut back to a leaf node. If you want to learn more about pruning, MSU offers pruning classes at different times of the year (generally during the fall or winter). You can sign up for newsletters to learn more about gardening through our website.